1. Field of the Invention
This invention relates to a method and apparatus for producing a corrugated tube made of synthetic resin, and more particularly to a method and apparatus for manufacturing a corrugated tube having a smooth inner wall surface along the length of the tube and a corrugated outer wall in longitudinal cross-section to provide enhanced flexibility and high pressure resistance.
2. Prior Art
Various types of flexible hose or tubes made of synthetic resin and method and apparatus for manufacturing the same have been heretofore proposed and some of them are currently in use.
For example, U.S. Pat. No. 4,292,113 discloses a method and apparatus for continuously forming a corrugated tube from a strip of synthetic resin in a softened state by the use of a plurality of rotating rollers arranged in a circular array such that said rollers define an imaginary cylinder about its central axis with the axes of said rollers being parallel to each other but angled relative to the central axis of the imaginary cylinder whereby as the rollers rotate the strip is helically wrapped around and advanced axially along said imaginary cylinder with portions of adjacent turns of the strip overlapping and joined together to form a continuous tube; wherein each of said rollers is provided with a plurality of axially spaced annular ridges therearound, said ridges of the respective rollers being so arranged that the associated successive ridges of each adjacent rollers are aligned in a spiral path around said imaginary cylinder.
However, by the aforementioned method it is not possible to manufacture a corrugated tube having a smooth inner wall surface. Thus, this method has a great disadvantage in that materials such as sands, stones or impurities contained in a fluid passing through the tube will pile up in the inner spiral grooves or corrugations inevitably formed in the interior of the corrugated tube, thus blocking the tube after long use.
Alternatively, U.S. Pat. No. 4,368,088 discloses a method for manufacturing a corrugated tube having a smooth inner wall surface along the length of the tube and a corrugated outer wall in logitudinal cross-section. Specifically, the method comprises the steps of arranging in a circular array a plurality of rollers each rotating at the same speed such that an imaginary cylinder is defined about its central axis by said rollers, the longitudinal axes of said rollers being parallel to each other but offset with respect to said central axis; continuously feeding a flat non-reinforced trip of synthetic resin material in a softened state generally perpendicuarly to the length of said rollers in such a manner that said strip is helically wrapped around and advanced axially along said imaginary cylinder with portions of adjacent turns of the strip overlapping one on another and welded together; continuously feeding a flexible core member and embedding it between the overlapping portions of adjacent turns of the strip to raise the upper one of said overlapping portions so as to define spiral raised convolutions around the tube; and pressing a pressure roller against the overlapping portions of the strip having said flexible core member embedded therein; whereby a corrugated tube is formed having spiral convolutions defined by the embedded flexible core member around the outer periphery of the tube along the length thereof and a smooth inner wall surface. However, the flexible tube produced according to this type of prior art method requires a reinforcing core member which is embedded in the overlapping portions of the strip as it is spirally wrapped around, in order to impart required flexibility and resistance to exterior pressure.
Thus, the tube having a core member embedded in its wall not only requires complicated manufacturing procedures but also has a disadvantage in that a uniform resistance to exterior pressure is not necessarily ensured throughout the tube wall due to differentials in strength between the core member-embedded portions and the remaining portions.